1. Field of the Invention
The present invention relates to an electroluminescent display device, and in particular, to transistors constructing the circuit structure in the pixel section of an electroluminescent display device.
2. Description of the Related Art
An electroluminescence (hereinafter referred to as EL) display device which uses an EL element which is a self-illuminating element as an illumination element in each pixel has attracted a strong interest as an alternative display device for a display device such as a liquid crystal display device (LCD) and a CRT because the EL display device has advantages such as thin width and low power consumption, in addition to the advantage of being self-illuminating. Such an EL display device has thus been researched.
In particular, there is a high expectation for an active matrix type EL display device in which a switching element such as, for example, a thin film transistor for individually controlling an EL element is provided for each pixel and EL elements are controlled for each pixel, as a high resolution display device.
FIG. 1 shows a circuit structure for one pixel in an active matrix type EL display device having m rows and n columns. In the EL display device, a plurality of gate lines GL extend on a substrate in the row direction and a plurality of data lines DL and power supply lines VL extend on the substrate in the column direction. Each pixel has an organic EL element 50, a switching TFT (first TFT) 10, an EL element driving TFT (second TFT) 20, and a storage capacitor Cs.
The first TFT 10 is connected to the gate line GL and data line DL, and is turned on by receiving a gate signal (selection signal) on its gate electrode. A data signal which is being supplied on the data line DL at this point is then held in the storage capacitor Cs connected between the first TFT 10 and the second TFT 20. A voltage corresponding to the data signal is supplied to the gate electrode of the second TFT 20 via the first TFT 10. The second TFT 20 then supplies a current, corresponding to the voltage value, from the power supply line VL to the organic EL element 50. In this manner, the organic EL element in each pixel is illuminated at a brightness based on the data signal, and a desired image is displayed.
The organic EL element is a current-driven element which is illuminated by supplying a current to an organic emissive layer provided between a cathode and an anode. The data signal output onto the data line DL, on the other hand, is a voltage signal with an amplitude corresponding to the display data. Thus, conventionally, in order to accurately illuminate the organic EL element by such a data signal, in an organic EL display device, a first TFT 10 and a second TFT 20 are provided in each pixel.
The display quality and reliability of the organic EL display devices described above remain insufficient, and the characteristic variations in the first and second TFTs 10 and 20 must be dissolved. In particular, reduction in characteristic variation in the second TFT 20 for controlling the amount of current supplied from the power supply line VL to the organic EL element 50 is desired, because such variation directly causes variation in the illumination brightness.
Moreover, it is preferable to construct the first and second TFTs 10 and 20 from a polycrystalline silicon TFT which has quick operation speed and which can be driven by a low voltage. In order to obtain a polycrystalline silicon, an amorphous silicon is polycrystallized by laser annealing. Because of various reasons such as, for example, energy variation in the irradiating laser at the irradiation surface, the grain size of the polycrystalline silicon is not uniform. When grain size is not uniform, in particular around the TFT channel, there is a problem in that the on-current characteristic or the like of the TFT may also vary.